There are generally two related art technologies for producing the high precision hollow cell array structure formed of plural fine components, which are (1) a photolithography and (2) an embossing process. However, in producing a honeycomb structure by photolithography, it is comparatively difficult to stably produce the honeycomb structure having an aspect ratio of 5 or more at relatively low cost. In producing a honeycomb structure by the embossing process, it is difficult to make all honeycomb walls thin (e.g., thickness of 10 μm or less with aspect ratio of 5 or more). FIGS. 20A through 20D illustrate steps of a method for producing a fine honeycomb structure (one-side openings) according to the related art. FIG. 20A illustrates the step of applying an aqueous gelatin solution 51 on a first substrate 52 having plural depressions 53, and reducing the ambient pressure on the obtained product (i.e., the first substrate 52 covered with gelatin solution 51). FIG. 20B illustrates the step of expanding a gelatin film 54 in a vertical direction due to pressure of gases held in the plural depressions 53 of the first substrate 52 by the gelatin film 54, and FIG. 20C illustrates a step of drying and cooling the resulting product of FIG. 20B under a reduced pressure, thereby obtaining a honeycomb structure 55. FIG. 20D illustrates the step of forming openings in a thin ceiling film (i.e., gelatin film) that faces a second substrate 57 by placing the honeycomb structure 55 that has been left at room temperature (e.g., about 20° C.) into a heating and humidifying container 56 heated at a temperature of 30° C. (higher than room temperature) with humidity of 80 to 90%. Since the temperature of the honeycomb structure 55 is lower than the temperature inside the heating and humidifying container 56, condensation is formed on a surface (i.e., thin ceiling film) of the honeycomb structure 55. FIG. 20E illustrates the step of forming openings in the thin ceiling film of a honeycomb structure 58 due to autogeneous shrinkage induced by its surface tension. Since the condensation is formed on the thin ceiling film of the honeycomb structure 55 and the rigidity of the thin ceiling film is lowered, openings are formed in the thin ceiling film of the honeycomb structure 55 due to autogenous shrinkage of the gelatin film induced by surface tension. The thin ceiling film of the honeycomb structure 55 is particularly thin around the center of each cell thereof, and therefore the openings form initially around the center of each cell. Condensation time is controlled in order to obtain desired opening shape corresponding to each cell. The honeycomb structure 55 is removed from the heating and humidifying container 56, and is thereafter dried in order to terminate the formation of the openings. In a case of the ceiling film having thickness of 0.05 μm, the openings are formed in 20 s.
Japanese Laid-Open Patent Application Publication No. 8-112873 (hereinafter also called “Patent Document 1”) discloses an example of a method for forming a hollow cell array structure by foaming. This technology intends to provide foam exhibiting excellent lightweight properties, insulating properties, and compressive strength. The disclosed technology includes uniformly forming a thermoplastic resin foam having high expansion ratio of 20 times in square hollows of cells of a thermoplastic resin member having a lattice-like horizontal section, thereby providing sheet foam. Japanese Laid-Open Patent Application Publication No. 10-80964 (hereinafter also called “Patent Document 2”) discloses a technology for producing a honeycomb structure; specifically, a technology for producing a hollow cell array structure having a stable quality with time. This technology provides a honeycomb structure formed of polyhedral column-like cells densely and three-dimensionally arranged in the resin member without having junctions between cell walls of the polyhedral column-like cells. The cells are obtained by systematically and three-dimensionally arranging foaming agents in the resin member and causing the foaming agents to generate bubbles in the resin member. However, this technology involves extending cell walls of the polyhedral column-like cells, so that the core portions of the polyhedral column-like cells are gradually becoming thin as the polyhedral column-like cells are extended. Accordingly, the cell walls of the polyhedral column-like cells may not be retained at a predetermined thickness with stability. That is, in the foaming step of this technology, it is important to cause foaming agents in spaces formed separately and systematically arranged in depressions of the resin member to generate bubbles simultaneously. If foaming agents in spaces formed separately and systematically arranged in depressions of the resin member generate bubbles sequentially in depressions of the resin member, individual bubbles may form spheres. Therefore, an intact honeycomb structure may not be obtained. In the above Patent Documents 1 and 2, a thermal foaming method is employed. However, with this method, if a temperature is not uniform over the entire resin member, time for foaming varies with different portions of the resin member. As a result, an intact honeycomb structure may not be obtained.
Japanese Examined Patent Application Publication No. 56-34780 (hereinafter also called “Patent Document 3”) discloses a technology for producing a honeycomb structure utilizing a stretching force of the opening operation of a platen. However, with this technology, it is also important but difficult to produce the honeycomb structure having uniform viscosity. In addition, Japanese Laid-Open Patent Application Publication No. 2007-98930 (hereinafter also called “Patent Document 4”) discloses a technology for producing a fine honeycomb structure having thin cell walls and oblong cells. The technology includes a first step of placing a first material having plastic deformation properties under a predetermined condition on a first substrate having plural separate depressions having spaces, and a second step of extending the first material having plastic deformation properties by inducing gas pressure in the spaces such that there are formed oblong hollow cells in predetermined directions.
Embodiments of the invention are devised in view of the above and other problems. That is, if such thin cell walls and cells forming a fine hollow cell array structure (i.e., honeycomb structure) are desired to be used as pixel cells of an image display apparatus such as an electrophoretic display, the fine hollow cell array structure needs to have uniform thin cell walls and cells into which an injecting material is injected in order to function as the display. However, in this technology, sizes of openings in the fine hollow cell array structure are determined based on the openings formed of fine depressions in the first substrate. Accordingly, if the sizes of the openings formed of fine depressions in the first substrate are small, an injecting material may not be easily injected into the openings of the fine cells of the fine hollow cell array structure. Conversely, if the sizes of the openings formed of fine depressions in the first substrate are large, a material forming the fine hollow cell array structure may accidentally be injected into the first openings formed of fine depressions in the first substrate, while covering the first substrate with the material forming the fine hollow cell array structure. As a result, it may be difficult to form uniform cell spaces in the depressions of the fine hollow cell array structure. If the hollow sizes of the depressions are various or the first material is accidentally injected into the openings of fine depressions in the first substrate, the expansion amount of gas in the fine hollow cell array structure changes. As a result, the fine hollow cell array structure having uniform cell walls and cells may not be obtained due to change in heights of the cells and thicknesses of the cell walls.
If the hollow cell array structure includes openings at both upper and lower surfaces, the above problem may be solved by forming the openings of the first substrate side larger than the openings of the hollow cell array structure side. However, it appears to be difficult to expand adjacent numerous separate hollows to form the hollow cell array structure, then remove a thin ceiling film formed over the cell walls of the hollow cell array structure, and immediately solidify the cell walls of the hollow cell array structure while maintaining an intact shape of the hollow cell array structure. With the above technology, an additional step of melting the thin ceiling film using solvent vapor may be required for removing the thin ceiling film over the cells of the hollow cell array structure, which results in increasing the production cost.